I found a site I wanted to share with everyone. This site is: BookCrossing.com. It's been around for a while, but I just stumbled across it for the first time today. It basically lets you put a label on your books (with an individualized tracking number) and "release" them by handing them to a friend or leaving them somewhere they're likely to be found and enjoyed. Found books may be lucky enough to get a journal entry to record their travels so that the path the book takes can be enjoyed by the original sender and subsequent finders.
Also, as a part of this idea, there are fairly regular book exchanges done as "book rings" or "book rays" where the books are passed from one person to another. (Ring books end up back at home, rays do not.) Though this is not the primary purpose of the site, it is certainly a fairly obvious modification of the original idea and happens pretty frequently from what I can tell.
Please visit by clicking this link and see what you think. If you choose to sign up, my referral name is "Evenstar75." I get nothing out of it but credit for having referred people (so don't feel pressured by the referral thing). I just think it's a very nice idea of a way to pass along the books that you no longer want.